1. Field of the Invention
The present invention relates to a surface plasmon resonance (SPR) sensor; and more particularly, to a surface plasmon resonance imaging sensor (SPRI sensor) capable of performing absolute calibration of refractive index (RI) change for each pixel on the image that represents either intensity or angle change of the reflected light, according to the variation of the surface plasmon resonance conditions, i.e., the changes in refractive index and/or optical thickness of material adjacent to the metal layer.
2. Description of Related Art
Surface plasmons are referred to as a quantized, collective oscillation of free electrons propagating along the surface of thin metal film. Surface plasmons can be excited by a TM-polarized light impinging on a metal film through a high refractive index material, e.g. a glass prism, above the critical angle. Critical angle is defined as the angle above which a total internal reflection (TIR) occurs. At a given energy (wavelength), surface plasmon wavevector is completely determined by optical properties of metal and dielectric material adjacent to the metal layer. If the wavevector of incident light matches well with that of surface plasmons, the energy of light is completely transferred to the surface plasmon mode. This is called surface plasmon resonance (SPR). An incident angle at which an SPR occurs (SPR angle) is very sensitive to the refractive index change of the material adjacent to the metal film. SPR sensor can perform quantitative as well as qualitative analysis, and measure the thickness and concentration of the material to be measured from the refractive index change of the material which is adjacent to the metal layer, using the above-mentioned characteristics.
An attempt to apply SPR sensor as a biosensor was proposed for the first time by C. Nylander and B. Liedberg in 1982 [C. Nylander and B. Liedberg, “Gas Detection by means of Surface Plasmon Resonance”, Sensors and Actuators 3, pp. 79-88, 1982]. Then, SPR sensor has been widely used as one of the typical non-labeling biosensor systems, which can measure the interaction of bio-molecules without a marker such as a fluorescent material. After the first commercialization by Bicore AB of Sweden in 1990, SPR sensor has been widely used as a typical non-labeled biosensor system by researchers in the field of bio-, chemical-, and biochemical industries (U.S. Pat. Nos. 5,641,640 and 5,965,456).
FIG. 1a is a cross-sectional view of a conventional angle-interrogated SPR sensor.
Referring to FIG. 1a, the conventional angle interrogated SPR sensor includes a transparent substrate 11, a metal layer 12 which usually vacuum evaporated on the transparent substrate 11, a prism 13 located under the substrate 11 to be optically coupled to the metal layer 12, a light source 14 emits light to the prism 13, and an optical detector 15 for detecting light reflected from the substrate 11. At this time, a material 16 to be measured is disposed on the metal layer 12. More importantly, to be coupled with surface plasmons, the incident light 14 must be TM-polarized, so at least one polarizer must be positioned on the optical path between light source 14 and optical detector 15.
Conventional SPR sensor usually measures the reflectivity of incident light, which goes through the prism coupler 13 as a function of angle and is sensitive to the surface plasmon coupling conditions. However, because the shape of incident light is point-like, it reflects only one point or one pixel on the upper surface of the transparent substrate 11 which is adjacent to the metal layer 12. Therefore, it is unpractical and time-consuming to illuminate all the surface point-by-point, to estimate or to calculate the thickness and/or refractive changes of spatially distributed sample surface 16.
In order to solve this problem, a surface plasmon resonance imaging (SPRI) sensor or a surface plasmon microscopy (SPM) was proposed for the first time in 1987 and in 1988, where the relative intensity difference of reflected light was measured for each pixel or point on the whole surface, at a fixed angle and wavelength. [Yeatman, E. and Ash, E. A., Electron Lett, 1987, 23, pp. 1091-1092, and Rothenhausler, B. and Knoll, W., Nature, 1988, 332, pp. 615-617].
FIG. 1b is a cross-sectional view of a conventional SPRI sensor.
Referring to FIG. 1b, the conventional SPRI sensor includes a transparent substrate 11, a metal layer 12 deposited on the substrate 11, a prism 13 located under the substrate to be optically coupled to the metal layer 12, a light source 14 for emitting light to the prism 13, and an optical detector 15 for the detection of light reflected from the substrate 11. At this time, an object 16 to be measured is disposed on the metal layer 12. More importantly, to be coupled with surface plasmons, the incident light 14 must be TM-polarized, so at least one polarizer must be positioned on the optical path between light source 14 and optical detector 15.
Conventional SPRI sensor exploits a spatially expanded and collimated light as a light source 14 to illuminate upper surface of the transparent substrate 11. To obtain a difference image of reflected light, which gives a measure of thickness and/or refractive index change for each pixel on the metal layer 12 at the same time, a two-dimensional light receiving device, e.g., a charge-coupled device (CCD) can be used to measure the intensity of reflected light from the substrate 11. Therefore it is possible to measure the relative thickness and/or refractive index changes of the whole surface of sample 16 simultaneously, which covers the metal layer 12.
Conventional SPRI sensor exploits a method of imaging the difference of reflected light, which is the relative difference in reflectivity for each pixel on the surface 11. The relative difference in reflectivity is caused by relative thickness and/or refractive index changes of the sample 16 for each pixel or each point of the sample surface, adsorbed on metal layer 12. However, because the magnitude of the difference in reflectivity strongly depends on the dielectric properties of material, i.e., refractive indices of the prism 13 and of metal layer 12, and also on the thickness of metal layer 12 at a given wavelength, it is not possible to estimate or calibrate absolute thickness and/or refractive index change of sample layer 16.